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This research focuses on the analysis of the free propagation of Hermite-Gaussian beams diffracted by a symmetrical annular aperture placed at the beam waist plane. The propagation is studied analytically and numerically using the angular spectrum method and the 2D fast Fourier transformation. Numerical simulation examples illustrate the propagation characteristics of the Hermite-Gaussian beams diffracted by an annular aperture. The beam truncation parameters and obscuration ratio influence Hermite-Gaussian beam diffraction properties. [ABSTRACT FROM AUTHOR]

It is well-known that a strong longitudinal electric field and a small spot size are observed when radially polarized beams are tightly focused using a high numerical aperture parabolic mirror. The longitudinal electric field component can accelerate electrons along the propagation axis at high intensities in the focal region, which opens an application in particle acceleration. In this paper, we present a rigorous derivation of the electric field obtained when a radially polarized, monochromatic, flat-top beam is focused by a parabolic mirror. The formulae were deduced from the Stratton–Chu integral known from vector diffraction theory. We examined the influence of the focusing parameters on the distribution of both the longitudinal and radial electric field components. In the small numerical aperture and short wavelength regimes, excellent agreement was found with the results obtained from the Rayleigh–Sommerfeld formula. The calculation method can be adapted for various beam types and for electromagnetic pulses as well. [ABSTRACT FROM AUTHOR]

Using the Richards–Wolf diffraction integral, the longitudinal energy evolution on the focal plane of the fractional order vector vortex (FOVV) beams was studied. These beams possessed a vortex topological charge n and a polarization topological charge m, and were subjected to tight focusing through a larger numerical aperture. Our investigation revealed the existence of backflow energy when the binary topological charges n and m satisfied the conditions of n + m = 2 or n − m = −2. The component circularly polarized vortex beams of  e − i 2 ϕ e ^ +  (i.e., the minus second-order vortex right circularly polarized beam) and  e i 2 ϕ e ^ −  (i.e., the second-order vortex left circularly polarized beam) played significant roles in the generation of reverse energy flux at the focal region. For FOVV beams with binary topological charges n and m, whose sum and difference were integers, the longitudinal energy on the focal plane exhibited axial symmetry. If the sum or the difference of the topological charges n and m was not an integer, the axisymmetric longitudinal energy on the focal plane was disrupted. [ABSTRACT FROM AUTHOR]

A label-free microscopy technology, dark-field microscopy, is widely used for providing high-contrast imaging for weakly scattering materials and unstained samples. However, traditional dark-field microscopes often require additional components and larger condensers as the numerical aperture increases. A solution to this is the use of a meta-surface slide. This slide utilizes a multilayer meta-surface and quantum dots to convert incident white light into a red glow cone emitted at a larger angle. This enables the slide to be used directly with conventional biological microscopy to achieve dark-field imaging. This paper focuses on the design and preparation of the meta-surface and demonstrates that using the meta-surface in a standard transmission optical microscope results in a dark-field image with higher contrast than a bright-field image, especially when observing samples with micron-sized structures. [ABSTRACT FROM AUTHOR]

The use of optical systems in medical imaging, computer electronics, large-scale industries, and space exploration is common. The performance of these devices is closely related to the compactness and fast responses of lenses that are used in these optical systems. Typical lenses suffer from several key issues, including limited efficiency, significant size, and the presence of diffraction-induced distortions that compromise their overall performance. Herein these limitations are addressed by designing and simulating an ultra-thin compact metalens also known as a flat lens using a dielectric metasurface. A 1D array of 31 nano-cylinders is placed on a glass substrate that is utilized for focusing the incident wave both on and off center in the focal plane using simulations. The nano-cylinders are comprised of amorphous silicon hydrogenated (a-Si:H), which has a varying radius in a 1D configuration. Amorphous silicon hydrogenated (a-Si:H) nano-cylinders are utilized for the manipulation of the phase of the incident beam working at a frequency of 474 THz. Three metalenses are introduced with focal lengths of 7.46 μ m, 10 μ m, and 12.99 μ m, each having a numerical aperture (NA) of 0.7, 0.6, and 0.5, respectively. The designed single-array metalens showed a transmission efficiency of 73%. The nano-cylinders obtained a full 0–360 phase control that is beneficial in focusing the beam at the center and beyond the center. Symmetric focusing is obtained in the case of off-center focusing on both sides of the optical axis. The design and simulations of the metalens are performed using finite difference time domain (FDTD) simulation tools. [ABSTRACT FROM AUTHOR]

Femtosecond laser processing technology offers a promising technique for the preparation of micro and nanostructures of single-crystal silicon carbide (SiC), thanks to its high precision and non-destructive processing. However, further research is needed to optimize processing parameters, as well as improve efficiency and quality of the process. This study conducts experiments to explore the effects of femtosecond laser ablation on single-crystal SiC. The influence and significance of parameters, such as fluence (F), repetition rate, scan speed (S), multipass scanning (c) and numerical aperture on the performance of grooves, including groove depth, groove width, heat-affected zone (HAZ) width, material removal rate (MRR) and side wall inclination angle, were studied. The results show that the influence of fluence and numerical aperture on groove depth, groove width, HAZ width, MRR and side wall inclination angle is very significant. The scan speed has a very significant effect on the groove depth, groove width, HAZ width and side wall inclination angle but has insignificant effect on the MRR. Repetition rate and multipass scanning have a very significant effect on groove depth, HAZ width, MRR and side wall inclination angle and a moderately significant effect on groove width. The experimental methods of increasing the aspect ratio and reducing the HAZ width were studied, and a significance analysis was carried out. Fluence, multipass scanning and z-layer feed have significant effects on groove depth, groove width, aspect ratio, HAZ width and MRR. The influence of polarization angle on groove depth, groove width, aspect ratio and MRR is insignificant. [ABSTRACT FROM AUTHOR]

Two-photon excitation in light-sheet microscopy advances applications to live imaging of multicellular organisms. In a previous study, we developed a two-photon Bessel beam light-sheet microscope with a nearly 1-mm field of view and less than 4-μm axial resolution, using a low magnification (10×), middle numerical aperture (NA 0.5) detection objective. In this study, we aimed to construct a light-sheet microscope with higher resolution imaging while maintaining the large field of view, using low magnification (16×) with a high NA 0.8 objective. To address potential illumination and detection mismatch, we investigated the use of a depth of focus (DOF) extension method. Specifically, we used a stair-step device composed of five-layer annular zones that extended DOF two-fold, enough to cover the light-sheet thickness. Resolution measurements using fluorescent beads showed that the reduction in resolutions was small. We then applied this system to in vivo imaging of medaka fish and found that image quality degradation at the distal site of the beam injection could be compensated. This demonstrates that the extended DOF system combined with wide-field two-photon light-sheet microscopy offers a simple and easy setup for live imaging application of large multicellular organism specimens with sub-cellular resolution. [ABSTRACT FROM AUTHOR]

The article discusses a breakthrough in video-rate mid-infrared photothermal imaging, enabling super-resolution imaging of biological systems in water. By detecting the transient photothermal signal induced by a single infrared pulse, the imaging speed has been increased by three orders of magnitude. This advancement allows for observing the dynamics of biomolecules in living organisms and mapping fat storage in free-moving organisms.

Metasurface optical devices have shown a tendency to gradually supersede the conventional bulk devices in applications requiring compactness and light weight. Typical metasurface-based flat lenses that manipulate the wavefront rely on phase mapping using a limited number of subwavelength structures. However, this approach greatly reduces the efficiency when limited structures are mapped for a high numerical aperture (NA). Here, we theoretically and experimentally propose low refractive index meta-grating-based multifunctional devices fabricated by three-dimensional printing technology. The meta-grating arrays effectively bend the incident polarized terahertz wave to the designed diffraction angle of 67°, with transmission efficiencies of 74.1% (S-dimer) and 54.2% (P-dimer) at the frequency of 0.1 THz. Then, the meta-lens based on the proposed meta-grating arrays achieves the measured focus efficiency of 32.1% with NA = 0.902, corresponding to a maximum collection angle of 64.5°. The designed flat devices with excellent characteristics in terms of efficiency and NA represent a new paradigm in high efficiency, low-cost and multifunctional terahertz devices. [ABSTRACT FROM AUTHOR]

We present a compact bichromatic imaging system, located outside of the vacuum chamber of a trapped ion apparatus that collects the fluorescence of 230.6 and 369.5 nm photons simultaneously on a shared electron-multiplying charge-coupled device (EMCCD) camera. The system contains two lens doublets, consisting of a sphere and an asphere. They provide a numerical aperture of 0.45 and 0.40 at 230.6 and 369.5 nm, respectively, and enable spatially resolved state detection with a large field of view of 300 μm for long 115In+/172Yb+ Coulomb crystals. Instead of diffraction-limited imaging for one wavelength, the focus in this system is on simultaneous single-ion resolved imaging of both species over a large field, with special attention to the deep UV wavelength (230.6 nm) and the low scattering rate of In+ ions. The introduced concept is applicable to other dual-species applications. [ABSTRACT FROM AUTHOR]

We demonstrate an axisymmetric inverse-designed metalens to improve the performance of zone-plate-array lithography (ZPAL), one of the maskless lithography approaches, that offer a new paradigm for nanoscale research and industry. First, we derive a computational upper bound for a unit-cell-based axisymmetric metalens. Then, we demonstrate a fabrication-compatible inverse-designed metalens with 85.50% transmission normalized focusing efficiency at 0.6 numerical aperture at 405 nm wavelength; a higher efficiency than a theoretical gradient index lens design (79.98%). We also demonstrate experimental validation for our axisymmetric inverse-designed metalens via electron beam lithography. Metalens-based maskless lithography may open a new way of achieving low-cost, large-area nanofabrication. [ABSTRACT FROM AUTHOR]

We propose a simple optical method and device design for the non-contact determination of small shift, thickness, refractive index, and tilt angle of thin films. The proposed sensor consists of a laser light source, a third- or two-order spiral amplitude zone plate with a high numerical aperture, and a CCD camera connected to a computer. It is shown that the third-order zone plate transforms the incident Gaussian beam into a three-petal rotating beam. By measuring the rotation angle of the three-petal intensity distribution, one can measure the following: a minimum shift along the optical axis of about 7 nm (the wavelength is 532 nm), a change in the plate thickness by 3 nm, a change in the tilt angle of the plate by 0.1 degrees, and a change in the refractive index by 0.01. [ABSTRACT FROM AUTHOR]

In synthetic diamond plates, the intrapulse-correlated dynamics of self-phase modulation and spontaneous nonresonant Raman scattering by center-zone optical phonons were for the first time directly investigated for tightly focused (focusing numerical aperture NA = 0.25) positively chirped visible-range high-intensity laser pulses with variable durations (0.3–9.5 ps) and energies transmitted through the sample. The observed self-phase modulation broadening and modulation of the transmitted light and Stokes Raman spectra for the (sub)picosecond pulse durations indicate the considerable Raman–Kerr contribution to the nonlinear polarization. The latter appears through plasma emission of the optical phonons, which emerges on the (sub)picosecond timescale and dominates at ≈1 ps. Later, this phonon contribution is eventually suppressed in the material due to picosecond-scale electron-lattice thermalization and the related thermally enhanced symmetrical decay of optical phonons into lower-frequency acoustic ones. [ABSTRACT FROM AUTHOR]

Quantifying free-form surfaces using differential confocal microscopy can be challenging, as it requires balancing accuracy and efficiency. When the axial scanning mechanism involves sloshing and the measured surface has a finite slope, traditional linear fitting can introduce significant errors. This study introduces a compensation strategy based on Pearson's correlation coefficient to effectively reduce measurement errors. Additionally, a fast-matching algorithm based on peak clustering was proposed to meet real-time requirements for non-contact probes. To validate the effectiveness of the compensation strategy and matching algorithm, detailed simulations and physical experiments were conducted. The results showed that for a numerical aperture of 0.4 and a depth of slope < 12°, the measurement error was <10 nm, improving the speed of the traditional algorithm system by 83.37%. Furthermore, repeatability and anti-disturbance experiments demonstrated that the proposed compensation strategy is simple, efficient, and robust. Overall, the proposed method has significant potential for application in the realization of high-speed measurements of free-form surfaces. [ABSTRACT FROM AUTHOR]

Compact visible wavelength achromats are essential for miniaturized and lightweight optics. However, fabrication of such achromats has proved to be exceptionally challenging. Here, using subsurface 3D printing inside mesoporous hosts we densely integrate aligned refractive and diffractive elements, forming thin high performance hybrid achromatic imaging micro-optics. Focusing efficiencies of 51–70% are achieved for 15μm thick, 90μm diameter, 0.3 numerical aperture microlenses. Chromatic focal length errors of less than 3% allow these microlenses to form high-quality images under broadband illumination (400–700 nm). Numerical apertures upwards of 0.47 are also achieved at the cost of some focusing efficiency, demonstrating the flexibility of this approach. Furthermore, larger area images are reconstructed from an array of hybrid achromatic microlenses, laying the groundwork for achromatic light-field imagers and displays. The presented approach precisely combines optical components within 3D space to achieve thin lens systems with high focusing efficiencies, high numerical apertures, and low chromatic focusing errors, providing a pathway towards achromatic micro-optical systems. Creating compact, lightweight and powerful optics that work well under visible light has been challenging. Here, the authors 3D print optically transparent polymers inside nanoporous glass in order to densely integrate refractive and diffractive elements, forming thin, high-performance hybrid achromatic imaging micro-optics. [ABSTRACT FROM AUTHOR]

While optical tweezers (OT) are mostly used for confining smaller size particles, the counter-propagating (CP) dual-beam traps have been a versatile method for confining both small and larger size particles including biological specimen. However, CP traps are complex sensitive systems, requiring tedious alignment to achieve perfect symmetry with rather low trapping stiffness values compared to OT. Moreover, due to their relatively weak forces, CP traps are limited in the size of particles they can confine which is about 100 μm. In this paper, a new class of counter-propagating optical tweezers with a broken symmetry is discussed and experimentally demonstrated to trap and manipulate larger than 100 μm particles inside liquid media. Our technique exploits a single Gaussian beam folding back on itself in an asymmetrical fashion forming a CP trap capable of confining small and significantly larger particles (up to 250 μm in diameter) based on optical forces only. Such optical trapping of large-size specimen to the best of our knowledge has not been demonstrated before. The broken symmetry of the trap combined with the retro-reflection of the beam has not only significantly simplified the alignment of the system, but also made it robust to slight misalignments and enhances the trapping stiffness as shown later. Moreover, our proposed trapping method is quite versatile as it allows for trapping and translating of a wide variety of particle sizes and shapes, ranging from one micron up to a few hundred of microns including microorganisms, using very low laser powers and numerical aperture optics. This in turn, permits the integration of a wide range of spectroscopy techniques for imaging and studying the optically trapped specimen. As an example, we will demonstrate how this novel technique enables simultaneous 3D trapping and light-sheet microscopy of C. elegans worms with up to 450 µm length. [ABSTRACT FROM AUTHOR]

Metasurfaces can modulate light with periodically arranged subwavelength scatterers, and they can generate arbitrary wavefronts. Therefore, they can be used to realize various optical components. In particular, metasurfaces can be used to realize lenses, so-called metalenses. In the last decade, metalenses have been actively studied and developed. In this review, we firstly introduce the fundamental principles of metalenses in terms of materials, phase modulation method, and design method. Based on these principles, the functionalities and the applications can consequently be realized. Metalenses have a much larger number of degrees of freedom compared with that of existing refractive or diffractive lenses. Thus, they afford functionalities such as tunability, high numerical aperture, and aberration correction. Metalenses with these functionalities can be applied in various optical systems such as imaging systems and spectrometers. Finally, we discuss the future applications of metalenses. [ABSTRACT FROM AUTHOR]

In this work, the image formation in a confocal laser scanning microscope (CLSM) is investigated for custom-made multi-cylinder phantoms. The cylinder structures were fabricated using 3D direct laser writing and consist of parallel cylinders with radii of 5 and 10 μ m for the respective multi-cylinder phantom, with overall dimensions of about 200 × 200 × 200   μ m 3 . Measurements were performed for different refractive index differences and by varying other parameters of the measurement system, such as pinhole size or numerical aperture (NA). For theoretical comparison, the confocal setup was implemented in an in-house developed tetrahedron-based and GPU-accelerated Monte Carlo (MC) software. The simulation results for a cylindrical single scatterer were first compared with the analytical solution of Maxwell's equations in two dimensions for prior validation. Subsequently, the more complex multi-cylinder structures were simulated using the MC software and compared with the experimental results. For the largest refractive index difference, i.e., air as the surrounding medium, the simulated and measured data show a high degree of agreement, with all the key features of the CLSM image being reproduced by the simulation. Even with a significant reduction in the refractive index difference by the use of immersion oil to values as low as 0.005, a good agreement between simulation and measurement was observed, particularly with respect to the increase in penetration depth. [ABSTRACT FROM AUTHOR]

Geometric-phase dielectric meta-lenses made of silicon with high numerical aperture and short focal lengths are fabricated and characterised. For circularly polarised light, the same meta-lens can act as a converging or diverging lens, depending on the handedness of the circular polarisation. This effect enables application for optical tweezers that trap or release µm-size polymer beads floating in a microfluidic channel on demand. An electrically addressable polarisation converter based on liquid crystals may be used to switch between the two states of polarisation, at which the light transmitted through the meta-lens is focused (trapping) or defocussed (releasing), respectively. [ABSTRACT FROM AUTHOR]

Second harmonic generation (SHG) microscopy is a commonly used technique to study the organization of collagen within tissues. However, individual collagen fibrils, which have diameters much smaller than the resolution of most optical systems, have not been extensively investigated. Here we probe the structure of individual collagen fibrils using polarization-resolved SHG (PSHG) microscopy and atomic force microscopy. We find that longitudinally polarized light occurring at the edge of a focal volume of a high numerical aperture microscope objective illuminated with linearly polarized light creates a measurable variation in PSHG signal along the axis orthogonal to an individual collagen fibril. By comparing numerical simulations to experimental data, we are able to estimate parameters related to the structure and chirality of the collagen fibril without tilting the sample out of the image plane, or cutting tissue at different angles, enabling chirality measurements on individual nanostructures to be performed in standard PSHG microscopes. The results presented here are expected to lead to a better understanding of PSHG results from both collagen fibrils and collagenous tissues. Further, the technique presented can be applied to other chiral nanoscale structures such as microtubules, nanowires, and nanoribbons. [ABSTRACT FROM AUTHOR]

Taking non-contact temperature measurements in narrow areas or confined spaces of non-uniform surfaces requires high spatial resolution and independence of emissivity uncertainties that conventional cameras can hardly provide. Two-color optical fiber (OF) pyrometers based on standard single-mode (SMF) and multi-mode optical fibers (MMF) with a small core diameter and low numerical aperture in combination with associated commercially available components can provide a spatial resolution in the micrometer range, independent of the material's emissivity. Our experiment involved using a patterned microheater to generate temperatures of approximately 340 °C on objects with a diameter of 0.25 mm. We measured these temperatures using two-color optical fiber pyrometers at a 1 kHz sampling rate, which were linearized in the range of 250 to 500 °C. We compared the results with those obtained using an industrial infrared camera. The tests show the potential of our technique for quickly measuring temperature gradients in small areas, independent of emissivity, such as in microthermography. We also report simulations and experiments, showing that the optical power gathered via each channel of the SMF and MMF pyrometers from hot objects of 250 µm is independent of distance until the OF light spot becomes larger than the diameter of the object at 0.9 mm and 0.4 mm, respectively. [ABSTRACT FROM AUTHOR]

Spatial biology is a rapidly growing research field that focuses on the transcriptomic or proteomic profiling of single cells within tissues with preserved spatial information. Imaging-based spatial transcriptomics uses epifluorescence microscopy, which has shown remarkable results for the identification of multiple targets in situ. Nonetheless, the number of genes that can be reliably visualized is limited by the diffraction of light. Here, we investigate the effect of structured illumination (SIM), a super-resolution microscopy approach, on the performance of single-gene transcript detection in spatial transcriptomics experiments. We performed direct mRNA-targeted hybridization in situ sequencing for multiple genes in mouse coronal brain tissue sections. We evaluated spot detection performance in widefield and confocal images versus those with SIM in combination with 20×, 25× and 60× objectives. In general, SIM increases the detection efficiency of gene transcript spots compared to widefield and confocal modes. For each case, the specific fold increase in localizations is dependent on gene transcript density and the numerical aperture of the objective used, which has been shown to play an important role, especially for densely clustered spots. Taken together, our results suggest that SIM has the capacity to improve spot detection and overall data quality in spatial transcriptomics. [ABSTRACT FROM AUTHOR]

Transmission window is a major concern in selecting the background material of photonic crystal fiber for supercontinuum generation. In this work, Zinc-Germanium Diphosphide (ZnGeP 2) based single material photonic crystal fiber has been proposed with high nonlinearity and low confinement loss for near and mid-infrared supercontinuum generation. As single material is used, hence the concerned difficulties for choosing separate materials for core and cladding is eliminated. Again, the light transmission window of ZnGeP 2 is very large (700 nm – 13000 nm). Optical properties like dispersion, numerical aperture, effective area, nonlinearity and confinement loss are analyzed and demonstrated for a wavelength range of 1000 – 5000 nm. COMSOL software is used to design the fiber and finite element method (FEM) is adopted for numerical analysis. The proposed fiber presents low and flattened dispersion in the range of 2250 – 5000 nm, nonlinearity of the order of 104 W–1km−1 and confinement loss of the order of 10–9 dB/m. The fiber is then used for supercontinuum generation in both near and mid-infrared range and an unprecedented bandwidth of 31000 nm has been achieved which may be pragmatic for outstanding medical applications. [ABSTRACT FROM AUTHOR]

This paper presents a transition taper for coupling light between optical fibers with different geometries and refractive index profiles used in Power over Fiber (PoF) systems. Global energy efficiency and costs are critical parameters when delivering high power to remote areas. High-power lasers have maximum coupling for large core fibers, while widespread multimode optical (OM1) fibers used in optical communications are cheaper. We study the optical losses between large core fibers (200 µm) and OM1 fibers (62.5 µm) theoretically and experimentally. We demonstrate that improvements of 2 dB can be obtained by adding the new tapered structure to the system, compared to the direct splice between both fibers. There is good agreement between measured and calculated loss values using a new Gaussian loss model to describe splices between tapered and straight fibers. The fabrication of the transition taper is also described. We also measure the numerical aperture (NA) changes in the downtaper zone and demonstrate that the lower the NA of the input light, the higher the efficiency improvement. [ABSTRACT FROM AUTHOR]

Edge couplers are widely utilized in photonic integrated circuits and are vital for ensuring efficient chip-to-fiber coupling. In this paper, we present a high-efficiency and compact polarization-insensitive multi-segment linear silicon nitride edge coupler for coupling to high numerical aperture fibers. By optimizing the thickness of the up cladding and introducing air slots in the transverse direction, we have further modified the limiting effect of the mode field. This innovative edge coupler scheme boasts a compact structure and is compatible with existing mature standard processes, with a total length of only 38 μm. We numerically demonstrate that the proposed edge coupler exhibits a low coupling loss of 0.22 dB/0.31 dB for TE/TM modes at λ = 1550 nm. Furthermore, the proposed coupler displays high wavelength insensitivity within the range of 1400–1850 nm and maintains a coupling loss of less than 0.2 dB with a manufacturing deviation of ±20 nm. [ABSTRACT FROM AUTHOR]

Extreme ultraviolet (EUV) radiation is a key technology for material science, attosecond metrology, and lithography. Here, we experimentally demonstrate metasurfaces as a superior way to focus EUV light. These devices exploit the fact that holes in a silicon membrane have a considerably larger refractive index than the surrounding material and efficiently vacuum-guide light with a wavelength of ~50 nanometers. This allows the transmission phase at the nanoscale to be controlled by the hole diameter. We fabricated an EUV metalens with a 10-millimeter focal length that supports numerical apertures of up to 0.05 and used it to focus ultrashort EUV light bursts generated by high-harmonic generation down to a 0.7-micrometer waist. Our approach introduces the vast light-shaping possibilities provided by dielectric metasurfaces to a spectral regime that lacks materials for transmissive optics. [ABSTRACT FROM AUTHOR]

Stimulated Brillouin scattering (SBS) microscopy is emerging as a promising approach for mechanical imaging in biological settings. It is based on a spectroscopic backscattering SBS setup, but with scanning of the sample and using higher apertures of the excitation and collection optics for adequate spatial sampling. Here, we provide direct experimental measurements and theoretical predictions of the aperture-induced spectral effects of SBS microscopy in water—a key constituent of biological systems. It is shown that with increasing numerical aperture (NA), the Brillouin frequency shift and peak gain decrease, while the Brillouin linewidth broadens asymmetrically with the commencing of an extended tail in the low frequency region for NA > ∼0.5. Further, significant distortions of the Brillouin spectral line shape are predicted for NAs close to 1, affecting the ability to retrieve spectral parameters of the Brillouin medium precisely and accurately. [ABSTRACT FROM AUTHOR]

Spatial frequency processing is an essential technology for extracting morphological information from an optical image. Although various Fourier-based flat optical elements have been proposed as spatial-frequency filters to realize image processing, their transfer functions are statically fixed once fabricated, thus limiting the versatile, dynamic functionalities and practical applications. Here, a novel practical tuning strategy is demonstrated to realize switchable spatial-frequency processing for edge-enhanced and bright-field imaging by employing a tunable hydrogel-scalable nanoslide. By utilizing multilayered metallic and hydrogel stacks construction, the nanoslide directly manipulates the optical spatial frequency in the wavevector domain and exhibits opposite image processing at different wavelength channels due to the cavity-induced wavelength-sensitivity. More intriguingly, via controlling the ambient humidity, the angular-dependent optical response of the nanoslide can be effectively tuned for dynamic edge-enhanced imaging due to the hydrogel's inflation from moisture. In addition, the nanoslide is readily fabricated at a large scale and integrated into compact imaging systems, such as a biomicroscope. With the advantages of a high numerical aperture -0.8, polarization-insensitive, microscopy-compatible, and facile architecture, the proposed hydrogel-based nanoslide can find potential applications in machine vision, real-time image processing, biological imaging, and analog computing. [ABSTRACT FROM AUTHOR]

To visualize the microscopic internal structure of cells, some conventional methods exist in the literature, such as irradiating fluorescent substances with light that maintains activation and light that suppresses activation to reduce the effective spot diameter or irradiating samples with moiré stripes of irradiating light and analyzing the pattern of the moiré stripes. This study proposes a high-resolution method to detect the intensity and optical path difference (OPD), representing the phase distribution reflecting the internal structure of the object, in a transmission image without using pigmentation for observing near-transparent objects such as cells or soft materials in liquids. First, the object is scanned using a laser beam. Second, the intensity owing to the interference between the 0th- and 1st-order diffracted light is detected using two photodetectors as electrical signals. The signals are separated into DC and AC components, with the intensity and phase separated through signal processing using their ratio. Two tilting optics are inclined toward each other at a symmetrical position with respect to the optical axis of the objective lens. The arrangement ensures that the incident diffracted light becomes part of the 0th-order diffracted light and ± 1st-order diffracted light at high spatial frequency. Thus, in contrast to conventional optical systems, high-resolution transmitted images can be obtained by acquiring high spatial frequency diffracted light. Moreover, the lens is placed close to the glass slide to inject the diffracted light with high spatial frequency into the lens of the oblique optical system. The proposed method obtains images of OPDs with a resolution higher than the reciprocal of the cutoff frequency of the numerical aperture of the objective lens used in the actual optical system. [ABSTRACT FROM AUTHOR]

We have developed a high magnification objective lens performed with apodized phase contrast microscopy for bright high contrast, aiming to image finer intact intracellular structures and to analyze organelles. It has a 100× magnification and a 1.3 numerical aperture. It has newly developed apodized phase plate for bright high contrast, with a 2%-transmittance phase ring and adjacent 8%-transmittance apodization areas. It weakens images of large phase objects, 3 µm in diameter for example, and enhances images of small objects. Apodized bright contrast images provide to detect wider range of phase differences than that of apodized dark contrast images. It will be useful for finer structure analysis of phase objects in life sciences. We obtained cell images enhanced inner details. It revealed motion of organelles such as the endoplasmic reticulum (ER), which was hard to observe in unstained cells. A transport along with the ER was clearly observed through the apodized phase contrast microscope by high-speed time-lapse imaging such as 1 frame/s or 10 frame/s. [ABSTRACT FROM AUTHOR]

A light-field camera is an optical system that provides multi-field-angle information in one shot. The microlens array (MLA) in a light-field camera system is the key to provide multi-field-angle information. However, its numerical aperture limits the microlens' spatial resolution and the image's quality. We proposed a new microlens array for the light-field camera in this work. This MLA consists of one larger primary microlens and 72 satellite microlenses, simultaneously providing high-resolution image information from different angles. Moreover, this MLA can be easily fabricated by nanoimprint at a low cost. The measured spatial resolution of the primary microlens is 53.74 lp/mm higher than satellite microlens when MTF = 0.3. Finally, a one-shot photograph of a light-field camera shows one high-resolution and other views from different angles of the bear doll. The high-resolution light-field camera has great potential to apply in object ranging and face recognition in the future. [ABSTRACT FROM AUTHOR]

Through filling CS 2 into the core, a liquid-core photonic crystal fiber(LCPCF) with a fully circular air hole structure was designed. The finite element method(FEM) and numerical analysis are combined to simulate the structure and optimize the parameters of the fiber. When λ = 1550 nm ,the geometric optimal parameters are Λ = 0.75 μ m, d 1 / Λ = 0.86 , d 2 / Λ = 0.96 , and d 3 / Λ = 0.20 . Meanwhile, the LCPCF can achieve a large negative dispersion of - 2697.06 ps/nm/km and a high nonlinearity of 50677.77 W - 1 km - 1 . The numerical aperture and light acceptance are 0.674 and 65.37%, respectively. By comparison, we believe that this LCPCF has obvious advantages in optical communication compensation and supercontinuum generation. [ABSTRACT FROM AUTHOR]

Pattern recognition techniques form the heart of most, if not all, incoherent linear shift-invariant systems. When an object is recorded using a camera, the object information is sampled by the point spread function (PSF) of the system, replacing every object point with the PSF in the sensor. The PSF is a sharp Kronecker Delta-like function when the numerical aperture (NA) is large with no aberrations. When the NA is small, and the system has aberrations, the PSF appears blurred. In the case of aberrations, if the PSF is known, then the blurred object image can be deblurred by scanning the PSF over the recorded object intensity pattern and looking for pattern matching conditions through a mathematical process called correlation. Deep learning-based image classification for computer vision applications gained attention in recent years. The classification probability is highly dependent on the quality of images as even a minor blur can significantly alter the image classification results. In this study, a recently developed deblurring method, the Lucy-Richardson-Rosen algorithm (LR2A), was implemented to computationally refocus images recorded in the presence of spatio-spectral aberrations. The performance of LR2A was compared against the parent techniques: Lucy-Richardson algorithm and non-linear reconstruction. LR2A exhibited a superior deblurring capability even in extreme cases of spatio-spectral aberrations. Experimental results of deblurring a picture recorded using high-resolution smartphone cameras are presented. LR2A was implemented to significantly improve the performances of the widely used deep convolutional neural networks for image classification. [ABSTRACT FROM AUTHOR]

Microlens arrays (MLAs) which are increasingly popular micro-optical elements in compact integrated optical systems were fabricated using a femtosecond direct laser write (fs-DLW) technique in the low-shrinkage SZ2080TM photoresist. High-fidelity definition of 3D surfaces on IR transparent CaF2 substrates allowed to achieve ∼50% transmittance in the chemical fingerprinting spectral region 2–5 μ m wavelengths since MLAs were only ∼10 μ m high corresponding to the numerical aperture of 0.3 (the lens height is comparable with the IR wavelength). To combine diffractive and refractive capabilities in miniaturised optical setup, a graphene oxide (GO) grating acting as a linear polariser was also fabricated by fs-DLW by ablation of a 1 μ m-thick GO thin film. Such an ultra-thin GO polariser can be integrated with the fabricated MLA to add dispersion control at the focal plane. Pairs of MLAs and GO polarisers were characterised throughout the visible–IR spectral window and numerical modelling was used to simulate their performance. A good match between the experimental results of MLA focusing and simulations was achieved. [ABSTRACT FROM AUTHOR]

Coupling light from a point source to a propagating mode is an important problem in nano-photonics and is essential for many applications in quantum optics. Circular "bullseye" cavities, consisting of concentric rings of alternating refractive index, are a promising technology that can achieve near-unity coupling into a first lens. Here we design a bullseye structure suitable for enhancing the emission from dye molecules, 2D materials and nano-diamonds positioned on the surface of these cavities. A periodic design of cavity, meeting the Bragg scattering condition, achieves a Purcell factor of 22.5 and collection efficiency of 80%. We also tackle the more challenging task of designing a cavity for coupling to a low numerical aperture fibre in the near field. Finally, using an iterative procedure, we study how the collection efficiency varies with apodised (non-periodic) rings. [ABSTRACT FROM AUTHOR]

Total internal reflection fluorescence (TIRF) illumination bypasses the axial diffraction limit of light by using an evanescent field to excite fluorophores close to a sample substrate. However, standard TIRF imaging through the objective requires a high numerical aperture (NA) to generate the evanescent wave. Available lenses have a high magnification with a correspondingly small field of view—ranging from ∼ 50 μm to 1 mm in diameter. Switching to the older prism-TIRF configuration introduced by Axelrod in the 1980s might seem to remove the requirement for high objective NA and allow the use of existing large-field objectives. Unfortunately, these lenses are unsuitable because their throughput of light is too low for TIRF imaging. As such, high sensitivity TIRF imaging over a much larger mesoscopic field has yet to be demonstrated. We have developed a prism-based TIRF illuminator for the Mesolens—a highly corrected objective lens with an unparalleled ratio of NA to magnification. The imaging field of the Mesolens is 204 times larger than that of the TIRF objectives previously described, increasing the optical throughput of the optical system by a factor of 25 compared to an off-the-shelf microscope objective of the same magnification. We demonstrate MesoTIRF imaging of cell specimens and show the multi-wavelength capability of the modality across more than 700 cells in a single image. [ABSTRACT FROM AUTHOR]

The ability to correlate optical hyperspectral mapping and high resolution topographic imaging is critically important to gain deep insight into the structure–function relationship of nanomaterial systems. Scanning near-field optical microscopy can achieve this goal, but at the cost of significant effort in probe fabrication and experimental expertise. To overcome these two limitations, we have developed a low-cost and high-throughput nanoimprinting technique to integrate a sharp pyramid structure on the end facet of a single-mode fiber that can be scanned with a simple tuning-fork technique. The nanoimprinted pyramid has two main features: (1) a large taper angle (∼70°), which determines the far-field confinement at the tip, resulting in a spatial resolution of 275 nm, an effective numerical aperture of 1.06, and (2) a sharp apex with a radius of curvature of ∼20 nm, which enables high resolution topographic imaging. Optical performance is demonstrated through evanescent field distribution mapping of a plasmonic nanogroove sample, followed by hyperspectral photoluminescence mapping of nanocrystals using a fiber-in-fiber-out light coupling mode. Through comparative photoluminescence mapping on 2D monolayers, we also show a threefold improvement in spatial resolution over chemically etched fibers. These results show that the bare nanoimprinted near-field probes provide simple access to spectromicroscopy correlated with high resolution topographic mapping and have the potential to advance reproducible fiber-tip-based scanning near-field microscopy. [ABSTRACT FROM AUTHOR]

Brain tumors, abnormalities, and malignancies are considered fatal. Early detection techniques could save lives if they are utilized effectively for brain cancers. They should have remarkable mobility, high accuracy, high response speed, and low losses. The presented study shows what is believed to be a step toward achieving these objectives. A novel biomedical photonic crystal fiber sensor that can accurately detect and discriminate between different brain cells is proposed. The anomalous brain cells include injuries, tumors, and malignant cells. The proposed sensor can detect these various types with high relative sensitivity and negligible losses in comparison to other photonic crystal fiber-based biomedical sensors. The presented sensor exhibits a relative sensitivity of 97.5%, an effective area of 4.17 × 10−8 m2, a negligible confinement loss of 1.76 × 10−11 cm−1, a remarkably low effective material loss of 0.005942 cm−1, and a numerical aperture of 0.3524. The presented sensor is eligible to work in the terahertz regimes with a wide range of 1–3 THz. Also, a detailed comparison between the presented sensor and associated photonic crystal fiber literature is carried out to authenticate the effectiveness and veracity of the presented structure. [ABSTRACT FROM AUTHOR]

The concept of metamaterials offers a flexible pathway to manipulate the macroscopic behavior of light by delicately designed microscopic subwavelength structures, which has been recently introduced to integrated photonics to create devices with ultra-compact footprint, excellent performance or versatile functionalities. However, the conventional design approach of metamaterials, including two separated steps of subwavelength structure design and the assembly of unit cells, often encounters challenges when facing extreme design targets. In this work, we propose a hierarchical inverse design approach by cascading a conventional unit-cell-based design with a holistic topology optimization. As a proof-of-concept, we demonstrate ultra-short-range light focusing and mode-size conversion enabled by on-chip meta-lenses. The shortening of tapering region pushes higher numerical aperture of on-chip lenses, leading to the violation of locally periodic approximation used in meta-lens design and thus poor device performance, which fortunately, can be well compensated by the follow-up holistic optimization step. We experimentally realize mode-size squeezing by almost 20 times in a tapering region as short as 8 μm and 5 μm with low insertion loss and broadband performance. The proposed design scheme provides practical guidelines to design metamaterials as flexible on-chip wavefront control and light routing devices for various applications in fiber communication, sensing and optical computing. [ABSTRACT FROM AUTHOR]

Nanoscale fluorescence emitters are efficient for measuring biomolecular interactions, but their utility for applications requiring single-unit observations is constrained by the need for large numerical aperture objectives, fluorescence intermittency, and poor photon collection efficiency resulting from omnidirectional emission. Photonic crystal (PC) structures hold promise to address the aforementioned challenges in fluorescence enhancement. In this review, we provide a broad overview of PCs by explaining their structures, design strategies, fabrication techniques, and sensing principles. Furthermore, we discuss recent applications of PC-enhanced fluorescence-based biosensors incorporated with emerging technologies, including nucleic acids sensing, protein detection, and steroid monitoring. Finally, we discuss current challenges associated with PC-enhanced fluorescence and provide an outlook for fluorescence enhancement with photonic-plasmonics coupling and their promise for point-of-care biosensing as well monitoring analytes of biological and environmental relevance. The review presents the transdisciplinary applications of PCs in the broad arena of fluorescence spectroscopy with broad applications in photo-plasmonics, life science research, materials chemistry, cancer diagnostics, and internet of things. [ABSTRACT FROM AUTHOR]

Nanophotonic phenomena, such as zero optical back scattering, nonradiating anapole states, etc. are related to the excitation of single dipolar modes—hence so far, they have only been observed within a few relatively high‐index dielectric materials (refractive index, n > 3.5) in the nanoscale regime at the optical frequencies. Here, dipolar regime is unraveled, close‐to‐zero backscattering is demonstrated, and optical anapoles are excited in mid‐index dielectric spheres (titanium di‐oxide, TiO2; n ≈ 2.6) at the mesoscale regime (particle diameter, d ≈ incident wavelength, λ) under illumination with tightly focused Gaussian beams (TFGBs). Successive scattering minima associated with dipolar excitation are observed satisfying the first Kerker condition in the scattering spectra of single TiO2 spheres with diameters in the micrometer range. Moreover, at specific wavelengths, the electric and magnetic dipolar scattering amplitudes of the dielectric microspheres simultaneously go close‐to‐zero, leading to the excitation of hybrid optical anapoles with a total scattering intensity ≈ 5 times weaker for TFGB illumination with numerical aperture, NA ≈ 0.95 compared to NA ≈ 0.1. The result pushes the boundary of the observation of nanophotonic phenomena to a new regime with regards to type and size of the materials. [ABSTRACT FROM AUTHOR]

Bessel beams are attaining keen interest in the current era considering their unique non-diffractive, self-healing nature and their diverse applications spanning over a broad spectral range of microwave to optical frequencies. However, conventional generators are not only bulky and complex but are also limited in terms of numerical aperture (NA) and efficiency. In this study, we experimentally develop a wavelength-independent Bessel beam generator through custom-designed metasurfaces to accomplish high resolution and large depth-of-focus imaging. These meta-axicons exhibit a high NA of up to 0.7 with an ability to generate Bessel beams with a full width at half maximum (FWHM) of 300 nm (~λ/2) and a depth of focus (DOF) of 153 μm (~261λ) in a broad spectral range of 500–700 nm. This excitation approach can provide a promising avenue for cutting-edge technology and applications related to Bessel beams for imaging along with a high axial resolution and an ultra-large depth of focus. [ABSTRACT FROM AUTHOR]

As with the decreasing feature size prompted by Moore's law and the continuous technological advancements in the semiconductor industry, the distortion of the projection lens is an important factor that affects the overlay. In this paper, we propose a wavefront-measurement-based method to detect the projection lens distortion in the lithographic system. By normalizing the coordinates of the Shack–Hartmann system with the image displacements represented in the Z2 and Z3 terms of Zernike coefficients, the offsets between the actual image points and the ideal image points can be determined. By offset collection at an array of 7 × 7 field points to establish an overdetermined system of equations, the proposed method can simultaneously detect the distortions of translation, magnification, rotation, decentering distortion, thin prism distortion, and third-order radial distortion. This distortion measurement method is highly flexible for distortion measurement with portable and compactly integrated sensors, enabling the real-time and cost-efficient measurement of wave aberration and distortion. For proof-of-concept experiments, a projection lens with a numerical aperture (NA) of 0.58 for i-line (365 nm) is used for experimental testing. The results reveal that the repeatability accuracy of distortion detection is 51 nm and the 72 h long-term reproducibility is 143 nm. [ABSTRACT FROM AUTHOR]

Femtosecond laser excitation of strongly absorbing thin films generates picosecond acoustic waves in the surrounding medium. In time-domain transient absorption experiments, these waves can give rise to Brillouin oscillations. In this contribution, the attenuation of the Brillouin oscillations in transient absorption microscopy experiments has been investigated for different excitation and detection conditions. The results show that the measured attenuation constants strongly depend on the numerical aperture (NA) of the microscope objective used in the experiments, with higher NA objectives giving larger attenuation constants. Modeling the effect using Gaussian beam theory shows that the increased attenuation at high NA arises from diffraction effects in both the light fields and the acoustic waves. These results are important for understanding the spectral resolution in imaging applications of the Brillouin scattering effect. [ABSTRACT FROM AUTHOR]

We present experimental demonstration of tilt-mirror assisted transmission structured illumination microscopy (tSIM) that offers a large field of view super resolution imaging. An assembly of custom-designed tilt-mirrors are employed as the illumination module where the sample is excited with the interference of two beams reflected from the opposite pair of mirror facets. Tunable frequency structured patterns are generated by changing the mirror-tilt angle and the hexagonal-symmetric arrangement is considered for the isotropic resolution in three orientations. Utilizing high numerical aperture (NA) objective in standard SIM provides super-resolution compromising with the field-of-view (FOV). Employing low NA (20X/0.4) objective lens detection, we experimentally demonstrate ∼ (0.56 mm × 0.35 mm) size single FOV image with ∼ 1.7- and ∼ 2.4-fold resolution improvement (exploiting various illumination by tuning tilt-mirrors) over the diffraction limit. The results are verified both for the fluorescent beads as well as biological samples. The tSIM geometry decouples the illumination and the collection light paths consequently enabling free change of the imaging objective lens without influencing the spatial frequency of the illumination pattern that are defined by the tilt-mirrors. The large and scalable FOV supported by tSIM will find usage for applications where scanning large areas are necessary as in pathology and applications where images must be correlated both in space and time. [ABSTRACT FROM AUTHOR]

The Schwarzschild microscope is suitable for sample navigation in secondary ion mass spectrometry (SIMS) because of its advantages of a simple structure, large working distance, and good coordination with the ion extraction system. The high numerical aperture (NA) of the microscope significantly reduces diffraction effects, but the resulting high-order geometric aberrations seriously affect the imaging quality. In this paper, a novel design method of the Schwarzschild microscope with a high NA (0.47) was proposed. The aberration distributions and compensation methods were investigated through tolerance analysis. The results showed that the tilt and decenter tolerances become the dominant factors limiting the spatial resolution, which could only be improved by ensuring the alignment accuracy of mirrors. Finally, the spatial resolution of the microscope in the home-built SIMS reached 2.19 µm. [ABSTRACT FROM AUTHOR]

The HEPS beamline software group is dedicated to the development, operation, and maintenance of unified experimental control and data acquisition software framework (Mamba, Figure 9) for high-throughput, multimodal, ultrafast, in-situ and dynamic experiments at HEPS. The High Energy Photon Source (HEPS) is a greenfield 4th-generation light source. HEPS Computing and Communication Group (HEPS-CC), provides the IT R&D and services for the facility and science communities, including IT infrastructure, network, computing, analysis software, data preservation and management, and public services. [Extracted from the article]

Two-photon excitation laser scanning microscopy (TPLSM) has provided many insights into the life sciences, especially for thick biological specimens, because of its superior penetration depth and less invasiveness owing to the near-infrared wavelength of its excitation laser light. This paper introduces our four kinds of studies to improve TPLSM by utilizing several optical technologies as follows: (1) A high numerical aperture objective lens significantly deteriorates the focal spot size in deeper regions of specimens. Thus, approaches to adaptive optics were proposed to compensate for optical aberrations for deeper and sharper intravital brain imaging. (2) TPLSM spatial resolution has been improved by applying super-resolution microscopic techniques. We also developed a compact stimulated emission depletion (STED) TPLSM that utilizes electrically controllable components, transmissive liquid crystal devices, and laser diode-based light sources. The spatial resolution of the developed system was five times higher than conventional TPLSM. (3) Most TPLSM systems adopt moving mirrors for single-point laser beam scanning, resulting in the temporal resolution caused by the limited physical speed of these mirrors. For high-speed TPLSM imaging, a confocal spinning-disk scanner and newly-developed high-peak-power laser light sources enabled approximately 200 foci scanning. (4) Several researchers have proposed various volumetric imaging technologies. However, most technologies require large-scale and complicated optical setups based on deep expertise for microscopic technologies, resulting in a high threshold for biologists. Recently, an easy-to-use light-needle-creating device was proposed for conventional TPLSM systems to achieve one-touch volumetric imaging. [ABSTRACT FROM AUTHOR]

In this paper, we design a novel photonic crystal fiber (PCF) chemical sensor wherein the cladding structure is made of a steering-wheel (SW)-shaped large non-circular air-hole. In this PCF, we introduce the rectangular air holes in the core and they are filled with any one of the analytes, namely, water, ethanol and benzene. We use the full-vectorial finite element method (FEM) to optimize the structural parameters. Based on the numerical results, we find that the proposed sensor exhibits a high relative sensitivity of 92.8%, 93.2% and 93.4% for water, ethanol and benzene, respectively, at THz. Furthermore, we study the various optical characteristics of the proposed PCF, namely, birefringence, effective area and numerical aperture (NA). [ABSTRACT FROM AUTHOR]

We propose a 2.1 µm high-energy dissipative soliton resonant (DSR) fiber laser system based on a mode-locked seed laser and dual-stage amplifiers. In the seed laser, the nonlinear amplifying loop mirror technique is employed to realize mode-locking. The utilization of an in-band pump scheme and long gain fiber enables effectively exciting 2.1 µm pulses. A section of ultra-high numerical aperture fiber (UHNAF) with normal dispersion and high nonlinearity and an output coupler with a large coupling ratio are used to achieve a high-energy DSR system. By optimizing the UHNAF length to 55 m, a 2103.7 nm, 88.1 nJ DSR laser with a 3-dB spectral bandwidth of 0.48 nm and a pulse width of 17.1 ns is obtained under a proper intracavity polarization state and pump power. The output power and conversion efficiency are 0.233 W and 4.57%, respectively, both an order of magnitude higher than those of previously reported holmium-doped DSR seed lasers. Thanks to the high output power and nanosecond pulse width of the seed laser, the average power of the DSR laser is linearly scaled up to 50.4 W via a dual-stage master oscillator power amplifier system. The 3-dB spectral bandwidth broadens slightly to 0.52 nm, and no distortion occurs in the amplified pulse waveform. The corresponding pulse energy reaches 19.1 µJ, which is the highest pulse energy in a holmium-doped mode-locked fiber laser system to the best of our knowledge. Such a 2.1 µm, high-energy DSR laser with relatively wide pulse width has prospective applications in mid-infrared nonlinear frequency conversion. [ABSTRACT FROM AUTHOR]